在MySQL中,学生表Students中包含sid(主键)、姓名sname。现在查询所有有重名的学生姓名,以下语句正确的是( )。 A. select sname from students group by sid having count(sid)>1; B. select sname from students group by sname having count(sid)>1; C. select distinct a.sname from students A, students B where A.sname=B.sname; D. select distinct a.sname from students A, students B where A.sname=B.sname and A.sid !=B.sid;